Thermography is concerned with materials which are substantially light-insensitive, but are sensitive to heat or thermographic. Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. A wide variety of chemical systems has been suggested some examples of which have been given on page 138 of the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), describing the production of a silver metal image by means of a thermally induced oxidation-reduction reaction of a silver soap with a reducing agent.
In U.S. Pat. No. 2,910,977 the following statement is made in the description in column 7, lines 23-27: "Stability towards exposure to light is improved by selecting highly purified materials; freedom from halides and sulfides is particularly important in the case of compositions involving silver salts." The disclosure in U.S. Pat. No. 2,910,977 concerned thermographic recording materials coated from solvent media.
WO 94/16361 discloses a multilayer heat-sensitive material which comprises: a colour-forming layer comprising: a colour-forming amount of finely divided, solid colourless noble metal or iron salt of an organic acid distributed in a carrier composition; a colour developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a colour-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the colour-forming layer.
Coating of thermographic recording materials from aqueous media is preferred over coating from solvent for ecological and economic reasons. However, the inventors of the present invention found that the choice of surfactants used in the production of thermographic recording materials using aqueous dispersions and solutions had a substantial effect upon the stability of the thermographic recording materials produced and on prints produced therewith.